Method and apparatus for measuring object thickness

ABSTRACT

A method and apparatus are provided for measuring the thickness of a test object. The apparatus includes an eddy current sensor having first and second sensor heads. The sensor heads are positioned to have a predetermined gap therebetween for passage by at least a portion of the test object through the gap. The sensor heads make measurements at given sampling locations on the test object as the test object is moved through the gap. The apparatus also includes a position sensing mechanism to determine positions of the sampling locations on the test object. The apparatus also includes an evaluation circuit in communication with the eddy current sensor and to the position sensing mechanism for determining the thickness of the test object at the sampling locations.

RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 11/522,416, filed on Sep. 18, 2006, which is a continuation ofU.S. patent application Ser. No. 10/685,210, filed on Oct. 14, 2003, nowU.S. Pat. No. 7,112,961, which claims priority from U.S. ProvisionalPatent Application Ser. No. 60/433,429, filed on Dec. 13, 2002, each ofwhich is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to devices for measuring thethickness of objects and, more particularly, to devices having eddycurrent sensors for measuring thickness.

2. Description of Related Art

Eddy current sensors are non-contact measurement devices used formeasuring the thickness of conductive objects. Briefly, an eddy currentsensor includes a sensor coil, which when driven by an AC current,generates an oscillating magnetic field that induces an eddy current inthe surface of a nearby conductive object. The eddy current is dependenton the strength of the magnetic B-field created by the AC current andthe impedance of the object, which is related to the thickness of theobject and the resistivity of the object. The thickness of the objectcan be determined based on the known resistivity of the object and themeasured eddy current or impedance.

In semiconductor manufacturing, one common use of eddy current sensorsis for measuring the thickness of a conductive layer (such as, e.g., acopper layer) deposited on a wafer substrate. Eddy current sensors areused for determining the thickness of a conductive layer at varioussampling locations on the wafer. In many cases, it is important to havea generally uniform conductive layer thickness to avoid problems insubsequent processing such as etching. It is accordingly important to beable to accurately determine the thickness of conductive layers so thatcorrective action can be taken, if needed, to obtain a desiredthickness. Alternatively, the wafer can be scrapped to avoid theunnecessary expense of further processing.

Currently available eddy current sensor devices for measuring thethickness of conductive layers on wafers are generally very slow. Thesedevices can also be very sensitive to inadvertent movement of the objectrelative to the eddy current sensors and, accordingly, often havecomplex and costly position control mechanisms in an attempt to providea generally uniform distance between the sensor and the wafer.

BRIEF SUMMARY OF EMBODIMENTS OF THE INVENTION

Methods and apparatus are provided for measuring the thickness of a testobject such as, e.g., a portion of a conductive layer deposited on awafer substrate. An apparatus in accordance with one or more embodimentsof the invention includes an eddy current sensor having first and secondsensor heads. The sensor heads are positioned to have a predeterminedgap therebetween for passage by at least a portion of the test objectthrough the gap. The sensor heads make measurements at given samplinglocations on the test object while the test object is moved through thegap. The apparatus also includes a position sensing mechanism todetermine positions of the sampling locations on the test object. Theapparatus also includes an evaluation circuit in communication with theeddy current sensor and with the position sensing mechanism fordetermining the thickness of the test object at the sampling locations.The apparatus can also include a mechanism for moving the test objectthrough the gap while the measurements are made.

In accordance with one or more embodiments of the invention, theapparatus also includes a displacement sensor for detecting anydisplacement of the test object in a direction generally extendingbetween the first and second sensor heads. The displacement sensor is incommunication with the evaluation circuit, which adjusts themeasurements of the sensor heads to compensate for any detecteddisplacement of the test object.

A method in accordance with one or more embodiments of the inventionincludes making measurements at sampling locations on the test objectusing first and second eddy current sensor heads positioned on oppositesides of the test object. The method also includes determining thepositions of the sampling locations on the test object, and calculatingthe thickness of the test object at the sampling locations. The testobject is moved relative to the sensor heads while making themeasurements.

In accordance with one or more embodiments of the invention, the methodalso includes the step of detecting any displacement of the test objectin a direction generally extending between the first and second sensorheads. The measurements can then be adjusted to compensate for anydetected displacement of the test object.

These and other features will become readily apparent from the followingdetailed description wherein embodiments of the invention are shown anddescribed by way of illustration. As will be realized, the invention iscapable of other and different embodiments and its several details maybe capable of modifications in various respects, all without departingfrom the invention. Accordingly, the drawings and description are to beregarded as illustrative in nature and not in a restrictive or limitingsense.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a representative eddy current sensorhead;

FIG. 2 is a block diagram of an apparatus for measuring the thickness ofan object in accordance with one or more embodiments of the invention;

FIG. 3 is a perspective view of certain components of the FIG. 2apparatus;

FIG. 4 is a schematic illustration of representative flux lines of asingle eddy current sensor in accordance with the prior art;

FIG. 5 is a schematic illustration of representative flux lines of thedual eddy current sensor heads of the FIG. 2 apparatus;

FIG. 6 is a graph illustrating the reduced sensitivity of an apparatusin accordance with one or more embodiments of the invention to changesin the distance between the test object and the eddy current sensorheads;

FIG. 7 is a graph illustrating representative values for distancecompensation factors in accordance with one or more embodiments of theinvention; and

FIG. 8 is a flow chart illustrating a process for measuring thethickness of an object in accordance with one or more embodiments of theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is generally directed to an on-the-fly eddycurrent sensor device for rapidly and accurately determining thethickness of a test object at various sampling locations on the object.Briefly, the device includes an eddy current sensor having two opposedheads that are spaced apart by a predetermined gap. During use, aportion of the test object is moved through the gap, and the thicknessof the test object is determined at various sampling locations on thetest object while the test object is being moved. The device alsoincludes a set of position sensors, which can be used to determine theposition of the sampling locations relative to the test object whenmeasurements are made.

Using two eddy current sensor heads on opposite sides of the test objectimproves the accuracy of measurements because the device issignificantly less sensitive to inadvertent movement or vibration of agiven sampling location toward or away from the sensor heads resultingfrom passage of the test object through the gap. The measurements can bemade on-the-fly, allowing multiple sampling locations to be quicklymeasured.

One or more embodiments of the present invention contemplate theinclusion of a Z-position displacement sensor to determine the distancebetween the test object and the sensor heads in order to determine anydistance related compensation factor to be applied to the raw data tocompensate for distance and vibration effects to even further improvemeasurement accuracy.

FIG. 1 schematically illustrates a representative eddy current sensorhead 10 that can be used in a thickness measurement device in accordancewith various embodiments of the invention. The eddy current sensor head10 includes a pot core 12 and a coil 14. By way of example, the core 12can be a split ferrite pot core. The core 12 can, e.g., have a diameterof about 9 mm and a height of about 4 mm. Cores having otherconfigurations and sizes can also be used. By way of example, the coil14 can comprise 26-32 gauge wire and have about 10-30 turns. Other wiresizes and coil configurations can also be used.

The sensor coil 14, when driven by an AC current, generates anoscillating magnetic field that induces an eddy current in the surfaceof the test object. The eddy current is dependent on the strength of themagnetic B-field created by the AC current and the impedance of theobject, which is related to the thickness of the object and theresistivity of the object. The thickness of the object can accordinglybe determined based on the known resistivity of the object and the eddycurrent detected by the sensor coil.

Other types of eddy current sensor heads can also be used. Theseinclude, e.g., sensor heads with two coils, in which a primary coil isdriven by an AC current and generates an oscillating magnetic field, anda secondary pickup coil receives a responsive signal from the testobject.

FIG. 2 is a representative block diagram of an apparatus 20 formeasuring the thickness of a test object in accordance with one or moreembodiments of the invention. FIG. 3 is a perspective view of someelements of the apparatus 20. Referring now to FIGS. 2 and 3, theapparatus 20 includes an eddy current sensor, which has two sensor heads24, 26 that can be connected in either a serial or parallel circuit. Thesensor heads 24, 26 are mounted on respective brackets 28 such that theyare spaced a predetermined distance from each other, forming a gate orgap therebetween. The gate distance can be varied depending on the sizeof the test object being measured. A typical range for use, e.g., insemiconductor manufacturing for measuring the thickness of layersdeposited on wafers can be between about 2-6 mm. Such a range has beenfound to provide suitable spot size, signal strength and handlingreliability in typical semiconductor processing applications.

The eddy current sensor heads 24, 26 can be connected to a sensor boardcircuit 30, which generates the AC current for driving the sensor heads24, 26 and which receives a pickup eddy current signal from the sensorheads 24, 26 indicative of the test object thickness. The pickup eddycurrent signal with voltage form is transmitted to a controller 32,which can include an analog to digital converter for converting thepickup signal to a digital signal for processing as will be describedbelow.

The AC current used to drive the coils can vary. By way of example, thedriving current can be at frequencies between about 300 kHz and 5 MHz.Other current values are also possible.

The device 20 also includes an array of position sensors 34, whichdetect the position of the test object 22 as it is moved through the gapbetween the eddy current sensor heads 24, 26. The position sensors 34are connected to the controller 32, which can determine the samplinglocations on the test object 22 when thickness measurements are made.One example of a position sensor that can be used in the array is anoptical sensor such as a through-beam type sensor. Examples of suitableposition sensors include the model EX-11 sensor commercially availablefrom SUNX of Japan.

To further increase measurement accuracy, one or more embodiments of thepresent invention contemplate the inclusion of a Z-position sensor 36 tomeasure the distance between the test object 22 and the sensor heads 24,26 in order to determine any distance related compensation factor thatcan be applied to the raw data to compensate for distance and vibrationeffects. One example of a suitable Z-position sensor is a laser distancesensor. An example of such a sensor is the model XZ-30V sensorcommercially available from OMRON of Japan.

The controller 32 computes the thickness of the test object 22 at thevarious sampling locations based on respective readings from thesensors. A representative controller 32 can include an analog to digitalconverter, a PLC (Programmable Logic Control) and a PC (personalcomputer). The analog to digital converter converts analog signals fromthe eddy current sensor and the Z-position sensor to digital form forprocessing. The PLC receives sensing signals from the sensors andperforms data logging or collection functions. The PC receives data fromthe PLC and performs measurement and compensation calculations. Themeasurement results can be output to an output device 33 such as, e.g.,a computer display or printer.

Various known methods can be used for computing the thickness of thetest object from the eddy current sensor readings. For example, one suchknown method uses empirical data of eddy current sensor readings takenof particular test objects having known thicknesses to generate sensorreading calibration curves. In use of the device, eddy current sensorreadings can be mapped to the calibration curves to determine thethickness of measured test objects.

By way of example, operation of the device 20 is now described fordetermining the thickness of a conductive layer on a wafer substrate 22.The wafer 22 is positioned on an end effector 38 connected to a roboticarm. The robotic arm is then actuated to move the wafer through the gateformed by the pair of eddy current sensor heads 24, 26. As the wafer 22moves through the gate, it passes the array of position sensors 34,which are successively tripped or actuated by the leading edge of thewafer 22. A sensing routine is triggered when the wafer 22 passes thefirst position sensor 34. The sensing routine can include the eddycurrent sensor taking periodic thickness readings (e.g., at a samplingrate of 1,000 readings/second), and the position sensors 34 detectingwhen the wafer edge passes each successive sensor to determine thevelocity of the wafer. Using this information, the controller 32 candetermine the measured thickness at each sampling location and theposition of each sampling location on the wafer. In this manner,thickness measurements can be taken along a given line extending acrossthe wafer. Measurements along different lines across the wafer can betaken, if desired, by rotating the wafer to a desired position and thenmoving it through the device 20 while making measurements.

The device preferably makes measurements on-the-fly, i.e., while thewafer is being moved through the gap between the sensor heads. Highsampling rates are possible, allowing the wafer thickness to be quicklymeasured. For example, and in accordance with one or more embodiments ofthe invention, a wafer having a diameter of about 300 mm can be measuredin about two seconds, at about 2,000 sampling points. Other samplingrates can also be used.

By using two eddy current sensor heads on opposite sides of the testobject, inadvertent movement of a given sampling location toward or awayfrom the sensor heads (resulting from movement of the test objectthrough the gap) does not significantly affect the measurement.Accordingly, more accurate measurements can be made at each samplinglocation. Also, the need for extensive positioning control mechanisms isavoided, and the measurements can be made more quickly. The sensorreadings can be continually made as the test object moves through thegap between the eddy current sensor heads.

By making quick and accurate measurements of the thickness of theconductive layer on the wafer, corrective action can be taken, ifneeded, to obtain a desired thickness. For example, if a generallyuniform thickness is desired and the measurements indicate that thethickness is not sufficiently uniform, the wafer can be subjected toselective chemical mechanical polishing or other processes to obtain thedesired uniform thickness.

FIG. 4 illustrates a representative set of flux lines generated by asingle eddy current sensor 50 as used in prior art thickness measurementdevices. The eddy current sensor generates a pattern of magnetic fluxlines. The test object intersects a plurality of the flux lines at agiven spacing from the eddy current sensor. If the test object 22 isinadvertently moved toward or away from the eddy current sensor, thenumber of flux lines intersected by the test object can changesignificantly even for small movements of the test object. As the numberof flux lines intersected by the test object changes, so does themeasurement reading of the eddy current sensor, reducing its accuracy.

FIG. 5 illustrates a representative set of flux lines generated by thedual eddy current sensor heads 24, 26 used in devices 20 in accordancewith the various embodiments described above. As shown, the test object22 can be moved toward or away from respective sensor heads 24, 26 witha significantly reduced change in the number of flux lines intersected.Accordingly, the device has reduced sensitivity to variations indistance between the test object and the eddy current sensor heads.

FIG. 6 illustrates an example of the differences in sensitivity todistance variations measured for two devices 24, 26, one having a singleeddy current sensor head and the other having dual eddy current sensorheads. The dimensions shown in FIG. 6 for the size of the sensors areprovided by way of example only. These dimensions can vary depending onthe particular application.

To even further increase accuracy of thickness measurements, the Z-axissensor can be used to compensate for inadvertent movement of the testobject in a direction between the sensor heads. The Z-axis sensor 36 candetect the distance between the test object 22 and the eddy currentsensor 24, 26 heads to determine a distance related compensation factorto be applied to the raw data generated by the sensors to compensate fordistance and vibration effects. FIG. 7 is a graph illustratingrepresentative compensation values that can be selected based on thedistance moved by the test object relative to the sensor heads. Thevalues in the graph were empirically determined, and can vary based onthe device used and the object being measured.

FIG. 8 is a flow chart generally illustrating a process for measuringthe thickness of a test object in accordance with one or moreembodiments of the invention. At step 100, a thickness measurement ofthe test object is made at a sampling location on the test object usingfirst and second eddy current sensor heads positioned on opposite sidesof the test object while moving the test object past the eddy currentsensor heads. At step 110, the position of the sampling location on thetest object is determined. At step 120, any displacement of the testobject in a direction generally extending between the first and secondsensor heads is detected. At step 130, the thickness of the test objectat the sampling location is calculated and adjusted, if needed, tocompensate for any detected displacement of the test object.

Having described preferred embodiments of the present invention, itshould be apparent that modifications can be made without departing fromthe spirit and scope of the invention.

1. An apparatus for dynamically measuring a thickness of a layer of awafer, comprising: an eddy current sensor having first and second sensorheads, said sensor heads positioned substantially opposite each otherand defining a predetermined gap therebetween for passage by at least aportion of said wafer, said eddy current sensor configured to cause saidfirst and second sensor heads to make measurements at one or moresampling locations on said wafer while said wafer is moving through saidgap; a robotic end effector configured to hold said wafer and move saidwafer linearly through said gap while said measurements are made so thatsaid measurements are made at said plurality of sampling locations alonga line across said wafer; and a controller connected to said eddycurrent sensor, said controller configured to determine said thicknessof said layer of said wafer at said plurality of sampling locations fromsaid measurements by said first and second sensor heads.
 2. Theapparatus of claim 1, further comprising a displacement sensor fordetecting displacement of the wafer in a direction generally extendingbetween the first and second sensor heads.
 3. The apparatus of claim 2,wherein said displacement sensor is connected to said controller, andwherein said controller adjusts said measurements of said sensor headsto compensate for detected displacement of said wafer.
 4. The apparatusof claim 2, wherein said displacement sensor comprises a laser distancesensor.
 5. The apparatus of claim 1, wherein each sensor head includes acore and at least one coil mounted therein.
 6. The apparatus of claim 1,further comprising an array of edge-detection sensors to successivelydetect an edge of the wafer as the wafer is moved through said gap. 7.The apparatus of claim 6, wherein said controller determines a velocityof said wafer from outputs of said edge-detection sensors.
 8. Theapparatus of claim 6, wherein said controller determines a position ofsaid sampling locations on said wafer from outputs of saidedge-detection sensors.
 9. The apparatus of claim 6, wherein eachedge-detection sensor comprises an optical sensor.
 10. A method ofmeasuring thickness of a conductive layer of a wafer, comprising thesteps of: moving the wafer linearly through a gap between first andsecond sensor heads of an eddy current sensor, the first and secondsensor heads positioned substantially opposite each other to define thegap; making on-the-fly measurements of said conductive layer at one ormore sampling locations along a line across said wafer using said firstand second sensor heads while moving said wafer; and calculating thethickness of the conductive layer at said sampling locations from saidmeasurements.
 11. The method of claim 10, further comprising detectingdisplacement of said wafer in a direction generally extending betweensaid first and second sensor heads.
 12. The method of claim 11, furthercomprising adjusting said measurements to compensate for saiddisplacement of said wafer.
 13. The method of claim 10, furthercomprising displaying data specifying said thickness of said layer ofsaid wafer at said sampling locations.
 14. The method of claim 10,further comprising successively detecting an edge of the wafer as thewafer is moved past said sensor heads with an array of edge-detectors.15. The method of claim 14, wherein detecting said edge of said waferincludes optically detecting said edge.
 16. The method of claim 14,further comprising determining a velocity of said wafer from outputsfrom the array of edge-detection sensors.